Law enforcement agencies often are confronted with hostage situations where armed intruders are barricaded inside a building. Officers on the scene generally have no means for determining the number and position of persons within the building, and are thus hampered in their efforts to resolve the situation. Similarly, law enforcement personnel planning a surprise raid on an armed compound would also greatly benefit from information related to the number and position of persons within. Such situational awareness decreases the amount of risk faced by the entering law enforcement personnel by decreasing the amount of unknowns. Furthermore, such a system would be of great use to rescue agency attempting to find survivors in cave-ins or collapsed buildings.
Prior attempts to provide law enforcement and rescue personnel with a priori knowledge of the occupants of a structure include acoustic, optical and infra-red (IR) detection systems. The acoustic solution is simply to have a very sensitive listening device (i.e. microphone), or array of them, and listen to determine if there were any noises coming from the room. However, without an array of directional devices, it is impossible to determine the location of the targets generating the sound. Furthermore, moving targets may not make enough sound to be detected.
The optical solution is to somehow, view the interior of the structure through a window, or to find a crack in the structure through which to view the interior, or actually drill a hole so that a camera of some sort could be inserted and the room surveilled. The drawbacks of this solution are that it takes time to find a crack or drill a hole and it is noisy to do so. Thus, in a hostage or raid situation, the law enforcement personnel could lose the tactical advantage of surprise by virtue of lack of stealth. Additionally, view through a window or crack may only provide a limited field of view, and so, parts of the room may be hidden. Moreover, if the room is smoke filled then this solution is ineffective. Finally, the IR solution is basically a thermal mapping solution. However this cannot be implemented as a through wall device, one must have a direct view of the room. Furthermore, for obvious reasons IR devices are rendered ineffective in fire-fighting scenarios.
On the other hand, ultra wideband (UWB) radars exhibit many desirable features that would be advantageous in those sorts of environments, such as high range resolution, low processing sidelobes, excellent clutter rejection capability, and the ability to scan distinct range windows. Additionally, the technique of time modulated UWB (TM-UWB) adds decreased range ambiguities and increased resistance to spoofing or interference. Impulse radar can operate on wavelengths capable of penetrating typical non-metallic construction material. These advantages make impulse radar particularly beneficial in short range, high clutter environments. Thus, impulse radars have beneficial applicability in environments where vision is obscured by obstacles such as walls, rubble, or smoke, and fire. Various embodiments of impulse radar have been described in co-owned U.S. Pat. No. 4,743,906, issued to Fullerton , May 10, 1988; U.S. Pat. No. 4,813,057, issued to Fullerton, Mar. 14, 1989; and U.S. Pat. No. 5,363,108, issued to Fullerton, Nov. 8, 1994, all of which are incorporated herein by reference. Moreover, arrays of such radars have been developed for such uses as high resolution detection and intruder alert systems, as described in co-owned U.S. Pat. No. 6,218,979B1, issued to Barnes, et al. Apr. 17, 2001; and U.S. Pat. No. 6,177,903, issued to Fullerton, et al Jan. 23, 2001, respectively, both of which are incorporated herein by reference. These systems benefit from being low-power, non-interfering, and yet capable of scanning through typical, non-metallic building material.
However, as indicated in the described patents, those implementations comprise two or more radar systems making them not easily transportable. The above-described scenarios benefit from ease of transport and stealth. Recent advances in ultra wideband radio technology have enabled the development of radar platforms that allow a single operator to detect and monitor targets through walls, rubble or other material.
A need, therefore, exists for a system that allows detection of moving targets through walls or other non-metallic building material, but capable of transport and operation by one user. Necessary to such a single-user system is a component for transmitting and receiving that will not jeopardize the operational suitability of the overall radar device, but at the same time, permit the scan of a wide field of view with high target resolution and minimal target ambiguities, or ghosts. Such a component must also enable the tracking of multiple targets in both azimuth and range in order to provide the best situational information to the user.
The present invention is directed to an antenna array that satisfies this need. The antenna array comprises a ground plane and a plurality of antenna elements. A version of the invention has the plurality of elements mounted onto the ground plane in two parallel rows, one row dedicated to transmitting signals, the opposing row dedicated to receiving signals. An alternative version of the invention comprises equal numbers of transmitting elements and receiving elements such that there are even pairs of transmitting and receiving elements.
A version of the invention employs an ultra wideband antenna element with a radially constant phase front. However, an alternative version uses antenna elements that do not exhibit a symmetric response in at least one plane. Thus, in this version, the transmitting elements are oriented 180xc2x0 in relation to a corresponding receiving element.
A further version of the invention seeks to achieve a uniform response pattern from all radiating and receiving elements by placing a parasitic material at each end of the transmitting and receiving rows. In one version of this alternative, the parasitic material is a dummy, or non-energized, antenna element. Still another embodiment, mounts the antenna elements to achieve horizontal signal polarization.
Another version of the present invention, mounts the elements to the ground plane such that inter-element spacing is substantially unique. Another version mounts the elements such that each element is off-set from its neighboring element in order to permit closer spacing, or, in the alternative, mounts the elements such that they are obliquely rotated with respect to neighboring elements.
Another version employs a reflective fence structure affixed to the ground plane between the transmitting and receiving rows. The fence structure may comprise a linear plane or it may comprise a curvi-linear surface. Another embodiment of the invention comprises a ground plane with curved or rolled edges.
A further version of the present invention uses antenna elements comprising feed tab structures. A version also comprises co-planar waveguides disposed upon the side of the ground plane opposite the side to which the antenna elements are mounted. A version has the co-planar waveguides in communication with the antenna elements through the ground plane at the feed tab structure. A further version comprises co-planar waveguides that are of substantially unique lengths.
A version of the present invention comprises a radome intended to overlay the array. Said radome may comprise a semi-cylindrical departure from the plane of the radome such that when over-laid on the array, the transmitting row fits with a hollow formed by the departure. Another version of the present invention also comprises a radome with acoustic bumpers mounted thereon, in addition to, or as, stand-offs.
A method for use of the array is also disclosed herein that overcomes a problem of element spacing to achieve reduced cross-range, or azimuthal, ambiguities. This method comprises the step of pairing a non-vertically aligned transmitting and receiving elements.